The problems are in the Hayes & Harlington area and will cause delays until further notice.
Technicians arrived on site at 4.20pm and are investigating the problem.
Passengers can use these alternative routes:
South West Trains services between London Waterloo and Reading / Windsor & Eton Riverside
CrossCountry between Reading, Oxford and Banbury
Chiltern Railways between London Marylebone and Banbury
London Underground via any reasonable route
You can follow this incident on Twitter using the hashtag #HayesHarlington or visit www.nationalrail.co.uk for the latest updated timetable.